The type specimen of Paranthropus robustus (TM 1517, including a partial cranium) was discovered at Kromdraai near Sterkfontein in 1938 and
described by Robert Broom as a new species. Subsequently, more robust australopithecines were discovered at the nearby site of Swartkrans.
These Swartkrans hominins were described by Broom as Paranthropus crassidens. Many palaeoanthropologists currently regard
the robust australopithecines from Kromdraai and Swartkrans as one species, but consensus has not been reached on this issue. A morphometric
analysis has been undertaken to assess the probability that specimens attributed to P. crassidens represent the same species as
that which is represented by TM 1517, the holotype of P. robustus. Our results failed to reject the null hypothesis that both sites sample
the same, single species of robust australopithecine.
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The type specimen of Paranthropus robustus (TM 1517, see Figure 1) was discovered at Kromdraai B in the Cradle of Humankind World Heritage
Site in South Africa and described by Broom1 as a new genus and species. Subsequently, additional specimens of ‘robust’
australopithecines were discovered at Swartkrans, within four kilometers of Kromdraai B, and described as a distinct species, P. crassidens.
2 Many palaeoanthropologists such as Tobias3, Brain4, Fuller5, and Kaszycka6, regard
the Kromdraai B and Swartkrans ‘robust’ australopithecine specimens as representing a single species. Others such as Grine
7,8,9,10 and Howell11, and more recently, Schwartz and Tattersall12, regard the two as distinct at a
species level, P. robustus and P. crassidens, respectively. To date, this issue has not been addressed by a morphometric analysis
of both cranial and dental material. The objective of this study is to test the hypothesis that the robust australopithecine crania from Swartkrans
and Kromdraai are morphometrically similar enough to belong to the same species, P. robustus
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Craniodental measurements were taken on original specimens: TM 1517a from Kromdraai, and SK 12, SK 46, SK 48, SK 79, SK 83, and SKW 11 from
Swartkrans. The set of cranial measurements was adapted from that of Lockwood13, and is listed in Table 1. Dental measurements
included only the maximum labiolingual/buccolingual breadth, because heavy interproximal wear obscures maximum mesiodistal lengths in most
specimens.5,14 Where a trait was present on both the right and left sides, the average of the two values was taken.
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Thackeray and colleagues15,16 described a method whereby measurements of pairs of specimens were compared to one another, in order to
assess probabilities of conspecificity. Measurements obtained from extant, conspecific male-female pairs were plotted against each other, with the
female on the x-axis and male on the y-axis. Morphometrically similar pairs tended to exhibit a relatively low degree of scatter
about a least-squares regression line associated with the equation y = mx + c. This degree of scatter, or degree of
dissimilarity, was quantified by calculating the standard error of the slope, m (here, “sem”). Using modern
conspecifics as a frame of reference, Thackeray and colleagues15 found that this sem statistic displayed a normal distribution
when log-transformed (base 10). Thus, pairs of specimens that were morphometrically very different from one another − specimens of different
species − can be expected to have a relatively high degree of scatter about the regression line, and so a relatively high sem.
Conversely, pairs of conspecifics tended to have a lower degree of scatter and hence a relatively low sem. Using an extant reference
sample of over 1 400 specimens of vertebrates and invertebrates, Thackeray16 presented a log sem value of -1.61 ±
0.23 as a statistical definition of a species expressed in terms of probabilities. The log sem method described by Thackeray and colleagues15 was developed further by Wolpoff and Lee17,18, who
used a statistic which they referred to as the ‘standard error test of the null hypothesis’ (STET). Like log sem, STET is
based on the standard error of the slope of the least-squares regression line in bispecimen comparisons. However, Wolpoff and Lee18
argued against the dampening effects of logging the sem statistic. Additionally, they noted that regressing specimen x on
specimen y produced different slopes and therefore different standard errors, as compared to regressions of y on x. Thus,
STET utilises the standard error of the slopes of both possible pair-wise regressions: STET = [(s.e.mx)2 +
(s.e.my)2]1/2, where mx and my are the least-squares slopes of each possible regression.
In this way, the STET statistic is invariant of how specimens are regressed on one another, and makes no assumptions about sex, which is often
difficult to determine in fossil specimens. Low STET values indicate very similar overall shape (low scatter about the regression lines). For this analysis, we used the STET statistic to assess the likelihood that TM 1517 and the Swartkrans crania represent the same species. If
TM 1517 represents a different species from that which is represented by the Swartkrans robust australopithecines, STET values computed for
Swartkrans specimens (compared against each other) should be significantly lower than the STETs derived from TM 1517-Swartkrans comparisons.
A ranked-sums test19 was used to determine whether the Swartkrans-only STET values were significantly different from the values computed
for the TM 1517-Swartkrans comparisons. This non-parametric test was appropriate in this case because the two samples were not independent:
The same Swartkrans crania were used to compute STET values for within-Swartkrans and TM 1517-Swartkrans comparisons. Finally, because the STET statistic is more reliable with increasing numbers of variables,18 the above procedure was repeated in a
secondary analysis, omitting pair-wise comparisons that share less than ten variables, as in the case of the following pairs: TM 1517-SK 12,
TM 1517-SK 79, SK 79-SK 12, and SK 12-SK 83. An advantage of the methods developed by Thackeray and colleagues15,16 and Lee and
Wolpoff17,18 is that they facilitate assessment of fossil assemblages when certain specimens do not preserve all of the traits under
study. Additionally, these methods provide a means of testing hypotheses about taxonomy, based on metric similarity. It should be noted that
these methods only test null hypotheses − hypotheses of no significant difference.
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The mean STET value for Swartkrans-only was 0.156 ± 0.118, and that of TM 1517-Swartkrans was 0.119 ± 0.104 (see Table 2).
The Wilcoxon test showed the distributions are not significantly different (p = 0.3) The TM 1517-Swartkrans distribution was within the
range of the Swartkrans distribution, with the exception of the TM 1517-SKW 11 comparison, which had a lower STET value than any comparison within
Swartkrans alone (see Figure 2). That the TM 1517-Swartkrans distribution had a lower mean STET value than the Swartkrans-only distribution,
highlights the morphological similarities of TM 1517 with many specimens from the Swartkrans sample.
Figure 2 shows that within the Swartkrans-only distribution there is a major outlier, the comparison of SK 12-SK79; this is one of the comparisons
preserving fewer than 10 variables in common. The secondary analysis, omitting the comparisons with less than 10 variables in common, produced
similar results to the whole-sample analysis (see Figure 3). The mean STET for Swartkrans-only was 0.120 ± 0.070, and that of TM 1517-
Swartkrans was 0.053 ± 0.012. In the reduced dataset, the TM 1517-Swartkrans STET values span an even smaller part of the lower range of the
within-Swartkrans variation, further underscoring the similarity between TM 1517 and the Swartkrans fossils. A Wilcoxon ranked-sums test on this r
educed dataset showed the TM 1517-Swartkrans and Swartkrans-only STET distributions to be significantly different (p = 0.02) at
the p < 0.05 level. Because the TM 1517-Swartkrans STET values were generally lower than those within Swartkrans, this significant result
is surprising, and further emphasises the morphological similarity between the crania from these two sites, as well as the great variation within
Swartkrans itself.
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The results show that one cannot reject the null hypothesis: That TM 1517 from Kromdraai and crania from Swartkrans sample the same species, Paranthropus (or Australopithecus) robustus. The TM 1517-Swartkrans STET values were generally lower than the within-Swartkrans values, indicating that not only is TM 1517 within the Swartkrans-only range of variation, but also that TM 1517 is generally more similar to Swartkrans crania than the Swartkrans crania are to each other. In fact, the lowest STET value from the analysis, indicating the two most similar specimens, is that of TM 1517 from Kromdraai and SKW 11 from Swartkrans Member 1’s Hanging Remnant. In light of the fairly large variation displayed by the Swartkrans fossils, our results point to a relatively stable lineage of robust australopithecine in the Sterkfontein valley in the lower Pleistocene. Our results indicate that the Kromdraai and Swartkrans robust australopithecines represent the same species. This is contrary to findings of Howell11, Grine7,9,10, and Schwartz and Tattersall12, whose conclusions were based largely on differences in dental sizes, proportions, and gross morphology between the two sites. Two issues arose with these authors’ conclusions. Firstly, the decisions to make specific distinctions appear to be have been made from gross inspection of each site’s summary statistics for given dental traits (e.g. M3 mesiodistal length), rather than from statistical tests. Along these lines, Fuller5 used a resampling procedure to compare the pooled Swartkrans-Kromdraai dental coefficients of variation (CV) to the CVs of modern humans, African apes, and fossil hominins.
Her results showed that variation within the pooled Swartkrans-Kromdraai sample was statistically no greater than in most of the modern and fossil
referents, leading her not to reject the hypothesis of a single species represented at the two sites. Secondly, some Kromdraai specimens contributing to the seemingly specific differences between the two sites may actually relate to the presence
of early Homo. For example, Grine9, 10 suggested that the teeth of a sub-adult from Kromdraai, KB 5223, represented a robust
australopithecine species different from that which was represented at Swartkrans. However, Braga and Thackeray20
have shown, using qualitative and quantitative criteria, that KB 5223 is probably attributable to early Homo. Placing all other Kromdraai B
dental specimens in the P. robustus sample along with Swartkrans specimens, these authors found it improbable that the KB 5223 postcanine
dentition came from the same P. robustus sample, but rather more likely indicated the presence of Homo at Kromdraai. In some
non-metric traits, this specimen is similar to the TM 1536 M1 and dm1, also from Kromdraai. However, the authors noted that
TM 1536 lacked the key diagnostic features of Homo molars, such as the absence of the hypoconulid on the dm1 and C6 on the
dm2, present in KB 5223. Thus, the results of Braga and Thackeray20 corroborated those of Fuller5, indicating
a high probability that the ‘robust’ dental specimens from Kromdraai and Swartkrans, with the exception of the KB 5223 dentition,
sample a single species. Our results further demonstrate the craniodental affinities of the Kromdraai and Swartkrans robust australopithecine
fossils.
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From our morphometric analysis, we failed to reject the null hypothesis, therefore there is a very high probability that the type specimen
of P. robustus, TM 1517, is the same species of robust australopithecine that is present at Swartkrans. Moreover, TM 1517 fit
comfortably within the range of craniodental variation seen in Swartkrans, and is indeed morphometrically very similar to many of the Swartkrans
fossils. Hence, the Swartkrans ‘robust’ australopithecines and TM 1517 can be considered conspecific, representing a single species,
Paranthropus (Australopithecus) robustus.
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Support for this research was provided by the Rackham Graduate School and the University of Michigan Department of Anthropology (ZC), and by the
French Embassy in South Africa, the National Research Foundation (South Africa) and the Andrew Mellon foundation (JFT). We thank Dana Begun and
Milford Wolpoff for helpful comments on earlier manuscripts, as well as two anonymous reviewers for helpful and insightful comments on an earlier
draft. Our gratitude also goes to Stephany Potze, Lazarus Kgasi and Tersia Perregil at the Transvaal Museum for helpful access to the fossil
material.
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